Polyimide, or more specifically aromatic poly (etherimide) such as the one sold under the brand name Kapton, is used widely in electronic and microelectronic fabrication because of its high temperature stability and low dielectric constant. This material, in the form of a film or tape, is used, for example, as the substrates of lead frames and so-called TAB tapes in IC chip packaging as well as the substrates of flexible printed circuits of more general application. The lead frames and circuits are metal-polyimide composites with the metal often applied directly to one or both sides of the substrate by known electroplating processes as described, for example, in U.S. Pat. No. 4,832,799, or by vacuum sputtering or electroless processes. The metal which ultimately forms the conductive traces or paths of the lead frame or circuit is usually copper, although there may be an intermediate layer of cobalt or nickel between the copper and the substrate to optimize the bond between the metal and the substrate.
Sometimes, gold, nickel or other metal is plated directly on a polyimide film substrate. Very often, specifications require that the metal component of the metal-polyimide composite, e.g. a lead frame, be plated with gold, gold alloy, nickel or other metal. Both processes require the immersion of the composite or substrate in an electroplating solution which typically contains ions of an alkali metal such as cesium, potassium or sodium to improve the conductivity of the solution.
Polyimide has a disadvantage in that its electrochemical activities often cause severe degradation of the polyimide structure at the metal-polyimide interface due to electrochemical reduction of the polyimide during the electroplating process. Although reduction of a polyimide is energetically disfavored because of the relatively low electron affinity of the bound oxygen atoms, the reduced species will be somewhat stable if surrounded with cations that serve to offset or mitigate the negative charge. Such reduction results in degradation or a weakening of the bond between the metal and the polyimide.
There are two criteria necessary for the degradation of the polyimide to occur. Firstly, the reduction potential during the electroplating process must be high enough to cause electrochemical reduction of the polyimide. Accordingly, this problem is particularly severe when electroplating a metal such as gold, gold alloy or nickel which requires a high cathodic potential. Secondly, as noted above, the electron charge of the reduced polyimide must be neutralized by cations in order for the reduction to continue. Otherwise, the buildup of the electric field by the accumulated negative charges in the polyimide will counter-balance the external voltage applied during the plating process and further reduction of the polyimide will stop.
Thus, the rate of the electrochemical reduction of the polyimide is dependent upon the mobility of the cations in the polyimide. As is known, the cation with the smaller hydrated ionic radius has a faster diffusion coefficient within the polymer. It turns out that the ions of the alkali metals such as cesium, potassium and sodium often present in the electroplating solution, as noted above, have small ionic radii. Therefore, those cations actually promote reduction of the polyimide and, therefore, contribute to the deterioration of the polyimide-metal bond.
It is possible to inhibit electrochemical reduction of the polyimide under such circumstances by adjusting the composition of the plating solution or the plating conditions such as the electrolyte PH or the plating current density. However those solutions to the problem may prevent using optimum plating conditions. In other words, the chemistry and/or conditions for optimum electroplating and the chemistry and/or conditions for minimum polyimide reduction may be mutually antagonistic.
Accordingly, the present invention aims to minimize polyimide-metal interface degradation of a polyimide-metal composite such as a lead frame or flexible circuit during electroplating by inhibiting electrochemical reduction of the polyimide during plating and without adversely affecting the plating process.